The present invention relates to the general field of metal finishing, more particularly to the provision of tin-lead coatings on metallic substrates such as copper, and still more particularly to aqueous acidic tin-lead immersion baths.
It has long been known that tin and lead can be co-deposited in non-electrolytic manner over metallic substrate surfaces such as copper by means of chemical exchange/replacement reaction using aqueous acidic solutions (baths) containing tin and lead salts. Such processes can find use in many metal finishing applications, and are in principle particularly suitable in sequences for the fabrication of printed circuits wherein tin-lead is used as an etch resist for predetermined areas of copper circuitry and/or wherein tin-lead is applied over selected copper areas (e.g., through-holes, surrounding pads, etc.) and later reflowed to provide a solder layer for preserving and/or enhancing the solderability of those areas.
Tin-lead immersion baths contain, as noted, sources of divalent tin and lead ions in the form of water-soluble tin and lead salts, and are strongly acidic, either by use of particular tin and/or lead salts (e.g., fluoborates) and/or by provision of acids or acid-engendering materials separate and apart from the tin and lead salts. Another essential component of tin-lead immersion baths is thiourea, serving the function as a complexing agent for the tin and lead ions in the bath. All known commercial tin-lead immersion baths also contain hypophosphite ion, e.g., hypophosphorous acid and/or alkali metal salts thereof, typically sodium hypophosphite, as an essential component for stabilizing the bath and providing denser, more uniform deposits.
Efforts to eliminate hypophosphite from tin-lead immersion baths have not proven successful in the art. In particular, elimination of hypophosphite from tin-lead immersion baths tends to result in deposits having a high tin concentration irrespective of the balance between tin and lead ions in the bath. This is problematic in situations where the bath is used to provide a tin-lead layer which is to be reflowed to produce a solder layer, since it is difficult, if not impossible, to obtain a layer whose concentrations of tin and lead are such as to provide the desired solder composition (i.e., most preferably at or near the 63% tin/37% lead eutectic) upon reflow. As a result, resort must be had to other soldering techniques, such as hot air solder levelling and the like, to achieve the desired aim of preserving and/or enhancing solderability of those areas.